Overview to C Language

11

2.1 Brief History of C

C’s ancestors include Combined Programming Language (CPL), Basic Combined Programming Language (BCPL), B, and ALGOL 68. CPL was developed at Cambridge University in the early 1960’s. BCPL is a simple systems language developed by Martin Richards in 1967 [SEBE96]. A systems language is a programming language used to create operating systems (like Windows and DOS) and compilers (like Visual Basic). ALGOL68 or Algorithmic Language, on the other hand, was developed for scientific applications. The first high-level language implemented under the UNIX operating system was designed and implemented in 1970 in Bell Laboratories by Ken Thompson. This was B, which was based on BCPL. Neither BCPL nor B is a typed language. Being untyped means that all data are considered machine words, which may be simple but leads to many complications. Because of this complication and several other problems, it led to the development of a new typed language based on B. Originally called NB (New B) but later named C, it was designed and implemented by Dennis Ritchie at Bell Laboratories in 1972. In some cases through BCPL, and in other cases directly, C was influenced by ALGOL 68. For more than a decade since C’s inception, the only standard was the book by Kernighan and Ritchie. Over time, several implementers added different features thus creating different versions of C. Between 1982 and 1988, ANSI produced a new official description of C which included many of the features that implementors had already incorporated into the language.

2.2 Types of Data

A C program is composed of a sequence of characters that are grouped by a compiler into identifiable tokens. These tokens can be classified as literals, identifiers, and keywords.

Timeline: 1963 1967 1968 1970 1972

CPL BCPL ALGOL68 B C

Chapter 2

12

2.2.1 Literals

Literals are classified under numeric and non-numeric literals. 2.2.1.1 Numeric Literals

Numeric literals can be further subdivided into whole numbers and real numbers. Whole numbers are also called integers. These types of numbers do not contain any fractional part and it should not have a decimal point. Real numbers, on the other hand, are also called floating point numbers. These numbers have fractional parts and it could also be expressed using scientific notation. Exponential representation may be used to represent real numbers. An exponential representation consists of an integer or real value followed by e then an integer value in the exponential base 10. Example. 2.5e2 is the same as 2.5 x 102, which is equivalent to 250.0 2e4 is the same as 2 x 104, which is equivalent to 20000.0 2e0 is the same as 2 x 100, which is equivalent to 2.0 2.e5 is the same as 2.0 x 105, which is equivalent to 200000.0 In defining numeric literals, the following rules should be followed:

1. No comma. 2. No space between the unary sign (+ or -) and the digits. 3. Must begin and end with a digit.

2.2.1.2 Non-Numeric Literals

Non-numeric literals may be in the form of a character or a series of characters. A series of characters is called a string. A string should be enclosed in double quotes, while a character is enclosed in single quotes. Some special characters may be used, but it should be preceded by the escape character \ (backslash). Example. ‘a’ ‘I’ ‘+’ ‘2’ “De La Salle University” “a string with double quotes \” within” “a single backslash \\ is in this string”

Overview to C Language

13

Other escape characters are listed below.

Escape Character Meaning \a alert \t tab \0 null character \’ single quote \% percent symbol

2.2.2 Identifiers

Identifiers are composed of a sequence of letters, digits, and the special character _ (underscore). Identifiers are defined by the programmer, thus they should be descriptive. Avoid using names that are too short or too long. Limit the identifiers from 8 to 15 characters only. Some rules must be followed for defining identifiers. They are as follows:

1. It must consist only of letters, digits, and underscores.

Correct Example Incorrect Example _duh x-1

thisisanidentifier2 num# a large num

num_1 y?

2. An identifier cannot begin with a digit.

Incorrect Identifier 1name 3to4

3. An identifier defined in the C standard library should not be redefined.

Incorrect Identifier printf scanf

4. Identifiers are case sensitive; meaning uppercase is not equal to the

lowercase. Example. ans ≠ Ans ≠ aNs ≠ anS ≠ ANs ≠ ANS

Chapter 2

14

2.2.2.1 Variables Variables are identifiers that can store a value that can be changed. These can be of different data types. They can store literals or some type of structured data. Structured data can contain one or more values or data.

2.2.2.2 Constants

Constants are identifiers that can store a value that cannot be changed. Usually, constant identifiers are all capital letters with underscores between each word to distinguish them from variables.

2.2.3 Keywords

Keywords are words that have a strict meaning in C. These are special words “reserved” by the programming language for processing, thus these may not be redefined by the programmer. Some keywords are listed below.

auto double int struct break else long switch case enum register typedef char extern return union const float short unsigned continue for signed void default goto sizeof volatile do if static while

2.3 C Expressions

In the C language, expressions may either be arithmetic or logical, but the result would be an integer or a real value.

2.3.1 Arithmetic Expressions

The following table will show the different operators and their uses. The rule shows the type of operands where the operator can be used and the resulting data type given the operands.

Overview to C Language

15

Operator Meaning Rule Example

+ addition integer + integer = integer integer + real = real real + integer = real real + real = real

5 + 2 = 7 5 + 2.0 = 7.0 5.0 + 2 = 7.0 5.0 + 2.0 = 7.0

- subtraction integer - integer = integer integer - real = real real - integer = real real - real = real

5 - 2 = 3 5 - 2.0 = 3.0 5.0 - 2 = 3.0 5.0 - 2.0 = 3.0

* multiplication integer * integer = integer integer * real = real real * integer = real real * real = real

5 * 2 = 10 5 * 2.0 = 10.0 5.0 * 2 = 10.0 5.0 * 2.0 = 10.0

/ division integer / integer = integer integer / real = real real / integer = real real / real = real

5 / 2 = 2 5 / 2.0 = 2.5 5.0 / 2 = 2.5 5.0 / 2.0 = 2.5

% remainder integer % integer = integer 5 % 2 = 1 -5 % 2 = -1 5 % -2 = 1 -5 % -2 = -1

In evaluating arithmetic expressions, the following rules should follow:

1. Parenthesis First. Evaluate expressions that are enclosed in parenthesis. If there are nested parenthesis, evaluate from inside out.

2. Operator Precedence. Evaluation of operators should follow a hierarchy of priorities. Evaluate expressions with higher priority operators first. unary +, - (positive and negative) highest *, /, % binary +, - (addition and subtraction) lowest

3. Associativity Rule. If expression to be evaluated have operators that are in the same precedence level, evaluate the expression from left to right.

Chapter 2

16

Example. z = 8 a = 3 b = 9 w = 2 y = -5 z – ( a + b / 2 ) + w * -y

8 3 9 2 -5 4 7 5 10 1 11 2.3.2 Logical and Relational Expressions

In C, evaluation of logical and relational expressions return 0 for false and 1 (or any nonzero value) for true. The relational and equality operators are shown below.

Relational Operators Equality Operators < less than == equal

<= less than or equal != not equal > greater than

>= greater than or equal The following shows the logical operators and their corresponding truth tables.

Logical Operator Meaning Truth Table && and true && true = true

true && false = false false && true = false false && false = false

|| or true || true = true true || false = true false || true = true false || false = false

! not !(true) = false !(false) = true

Overview to C Language

17

Example. salary < MIN_SALARY || dependents > 5 temperature > 90.0 && humidity > 0.90 n >= 0 && n <= 100 !(0 <= n && n <= 100)

Rules in evaluating logical and relational expressions are similar with evaluating arithmetic expressions. These operators also follow precedence rules. The updated list is as follows: Operator Precedence ( ) highest !, unary +, - *, /, % binary +, - <, <=, >, >= ==, != && || lowest

Example. flag = 0 y = 4.0 z = 2.0 x = 3.0 !flag || ( y + z >= x – z ) 0 4.0 2.0 3.0 2.0 6.0 1.0 1 1 1 2.3.3 Converting Mathematical Formula to C Expression

To solve mathematical problems using the computer, the formula should be translated to the programming language to be used. In this case, arithmetic operations should be in C expressions. Example.

b2 – 4ac b * b – 4 * a * c a+b c+d (a + b) / (c + d)

1 1+x2 1 / (1 + x * x)

a x –(b + c) a * -(b + c)

Chapter 2

18

2.3.4 Converting English Conditions to C Expression

Solutions to problems may sometimes depend on a set of conditions. To use the computer to solve such problems, these conditions should be converted to the C. Example.

x and y are greater than z x > z && y > z x is equal to 1.0 or 3.0 x == 1.0 || x == 3.0 x is in the range of z to y, inclusive z <= x && x <= y x is outside the range of z to y !(z <= x && x <= y)

z > x || x > y Self Evaluation Exercises 1. Determine if the following identifiers are valid or invalid.

a) L8r b) star*ting c) num_Values d) 4u e) one_i_aren’t

2. Evaluate the following expressions a.) 10 + 23 % (17 – 4 * 2) / (24 – (7 + 15 % 2)) b.) 150 - (-6 + 8 * 4 – 22 % 4) – (5 – (15.2 / 2)) c.) (7 == 7.0) && ( 15 > 3) || !((7 >4) || (7 > 3)) d.) (8 > 13 % 3) || (7 > 22 % 3) && (5 == 30 / 6)

3. Convert the following mathematical equations to C expressions without adding

unnecessary parenthesis a.) 1 + X 1 + 1 6 8 b.) (X)(Y)(Z)

(X2 – Y2) + Y 4 + X 2 – Z

4. Convert the following statements to C expressions a.) X is neither 6 nor 8 b.) X is any number except 1, 2, and 3 c.) REVENUE is at most 80% of SALES d.) contestant’s HEIGHT is at least 175 cm and AGE is between 18 and 23,

inclusive e.) X is between 100 and 200, exclusive, except 120, 130, and 180

Overview to C Language

19

5. Write the C statement that will convert an amount in dollars to its peso equivalent. Assume that PhP1 is equal to $51.75.

Chapter Exercises 1. Determine if the following identifiers are valid or invalid.

a) 3id b) 1_i_am c) R3D3 d) int e) per-capita f) o_no_o_no g) me_to-2 h) xYshouldI i) phone# j) MAX_SPEED k) G l) __yes

2. Determine if the following are valid or invalid whole number literals.

a) -10500 b) 435 c) 2,020 d) +50,000 e) 21.5

3. Determine if the following are valid or invalid real literals a) 2.34e2 b) 15e-0.3 c) 125 d) 34,500.99 e) 0.005

4. Determine if the following are valid or invalid character literals.

a) ‘M’ b) ‘n1’ c) ‘\\’ d) ‘”’ e) ‘+’ f) ‘&’

5. Given x = 2.0 and y = 3.0, evaluate the following:

a) 2 – 4 * 3 + 26 / 2 b) (3 + 4) * x / 2 + y c) 5 + 6.6 / 2.2 * 0.5

Chapter 2

20

6. Given i = 1, j = 2, k = 3, x = 5.5, and y = 7.7, evaluate the following whether they

yield a value of TRUE/FALSE: a) i < (j – k) b) (x – y) <= ((j – k) – 1) c) (k + j) != (i + 1 * 4) d) ! (1 == 1) e) i && j f) i == j && i + j == k || y == x + 2 g) –i <= j – k && !j

7. Assume the values a = 1, b = 2, c = 0, d = 5.0, and e = 25. What is the output of the

following: a) a + b * c && a b) 3 / a + b / e || c c) 10 + 15 || 0 && 5 > 3 + 3 d) 1 + 2 > 3 * 4 || 5 && 3 > 4 == 0 e) 1 % 2 + 1 == 0 + 1 && 2

8. Convert the following conditions to C expressions:

a) Commission = (sales – sales X .10) * .25 b) Commission = (sales – sales X 10/100) * 25/100 c) Interest = Amount X Rate d) Semiannual Interest = 600/10% e) age is from 18 to 21 inclusive f) water is less than 1.5 and also greater than 0.1 g) year is divisible by 4 h) speed is not greater than 55 i) y is greater than x and less than z j) w is either equal to 6 or not greater than 3

9. Write the C statement that will compute for the area of a triangle given the base and

the height. 10. Write the C statement that will convert a Fahrenheit (F) measure to a Celsius (C)

measure. (C = 5/9 x (F – 32)) 11. Write the C statement that will convert an amount in peso to its dollar equivalent.

Assume that PhP1 is equal to $51.75. 12. Write the C statement(s) that will compute the least number of Php5 and Php1 coins

given an amount. Example: There are 3 Php5 and 2 Php1 in Php17.